Manus.

The astrolabe encoded the sky in geometry. The hand is a geometry, of the body's own making. Hinge joints at every phalanx, ball joints at the thumb, the carpals tessellating in two rows like a stone wall built by the same mason who built the wrist. The hand is the part of the body that articulates other instruments; it is therefore the instrument that makes instruments.

Galen of Pergamon was a Greek physician who became chief medical officer to the gladiators of Pergamon and then physician to Marcus Aurelius. Roman law forbade human dissection; he worked from Barbary macaques. The hand he described is approximately a human hand, but with the differences a macaque's hand has — and because Galen wrote in Greek and was translated into every language of medicine afterwards, his version of the hand dominated Western anatomy for thirteen hundred years.

Andreas Vesalius was a Flemish anatomist working in Padua in the 1530s. He dissected human cadavers — sometimes illegally — and discovered that Galen had been wrong about more than two hundred details. He published De Humani Corporis Fabrica in 1543, illustrated by an artist trained in Titian's workshop. The hand plates in Book II are the first anatomically correct drawings of a human hand in the Western tradition. The substrate of medicine had changed from the macaque to the human; the discipline rebuilt itself around what was actually there.

Bernhard Siegfried Albinus worked at Leiden two centuries after Vesalius. With the engraver Jan Wandelaar he produced Tabulae sceleti et musculorum corporis humani — the most beautiful anatomical plates ever made. Each figure shows the muscles peeled back in successive layers against a landscape background; rhinoceroses and trees behind the dissected figures, because Albinus thought the muscle layer needed beautiful company. The hand in his plates is rendered with the patience of a portraitist and the precision of a surgeon. The aesthetic is what we are still chasing.

MRI replaced cadaver dissection; the 3D model replaced the engraved plate; the surgical robot replaced the surgeon's hand. What didn't change is the proposition: the hand can be understood by drawing it accurately, layer by layer, naming each structure in Latin so that any physician in any century reads the same atlas. The naming of bones is older than the printing press and outlasts every reproduction technology that printed them.